Cytokinins (N6-substituted adenine derivatives) are a group of plant homones that stimulate cell division in plant tissue cultures and in plant tumors (crown gall). They also affect growth of transformed animal cells, being stimulatory under certain conditions and inhibitory under others. Very recently, it has been shown that cytokinins stimulate DNA replication in a fibroblast (BHK) cell line where DNA replication has been blocked by an inhibitor of mevalonate synthesis. Cytokinin-type bases also occur as constituents of certain microbial, plant and animal tRNA species. In this grant request, we propose to elucidate the site of action of cytokinins. For this purpose, we shall synthesize the cytokinin photoaffinity label 8-axido-N6-benzyladenine in radioactive form. This compound will be used to localize and characterize cytokinin receptors in two organisms, mass protonemata and cultured tobacco cells. In moss protonemata, cytokinins induce gametophyte bud formation within 16-24 h of hormone application. Protonemata are haploid, filamentous structures and can be cultured much like fungi. Mutants are available that do not respond to cytokinins and that may have impaired receptor functions. Cytokinin binding in target cells of protonemata will be demonstrated by autoradiography. Receptor protein(s) will be isolated from homogenates of protonemata and tobacco cells and characterized by methods of organelle and protein fractionation and purification. According to available evidence, cytokinins are regulators of cell division in plant and animal cells. Elucidation of the site and mode of action of cytokinins may yield results that will help explain the mechanism(s) by which cell division is regulated in normal and tumorous plant and animal cells.